(1) Field of the Invention
This invention relates to a method of stripping unreacted materials from a urea synthesis effluent intermediately obtained in a urea synthesis process from ammonia and carbon dioxide. More particularly, it relates to a method of stripping unreacted materials in a urea synthesis process which comprises first bringing a urea synthesis effluent intermediately obtained from ammonia and carbon dioxide into contact with a gas containing carbon dioxide under adiabatic conditions or with a little cooling, followed by the conventional carbon dioxide stripping process.
(2) Description of the Prior Art
The urea synthesis reaction from ammonia and carbon dioxide is normally effected under the conditions of a temperature of from 170.degree. to 210.degree. C., a pressure of from 130 to 300 kg/cm.sup.2 (gauge pressure, and so forth) and an ammonia to carbon dioxide molar ratio of from 2.5 to 6.0, whereby from 40 to 80% of the carbon dioxide is converted to urea and the balance remains in the form of ammonium carbamate. The excess ammonia and ammonium carbamate contained in a resulting urea synthesis composition consisting of urea, water, excess ammonia and ammonium carbamate (hereinafter referred to as a urea synthesis effluent) are first separated as ammonia gas and carbon dioxide gas and a small amount of residual ammonia and ammonium carbamate contained in the aqueous urea solution are also separated in a subsequent step.
Ammonia and carbon dioxide thus separated are recovered in a gaseous state, or in the form of a condensate or of a solution thereof absorbed in water, a dilute aqueous ammonium carbonate solution, an aqueous urea solution, or the like to be recycled to the urea synthesis.
Several processes for separating excess ammonia and ammonium carbamate from a urea synthesis effluent as ammonia gas and carbon dioxide gas for purpose of recovery are known in the art. The so-called carbon dioxide stripping process, in which the urea synthesis effluent is brought into countercurrent contact with carbon dioxide gas under heating, is known as one of the typical examples of the conventional processes known in the art.
In accordance with the carbon dioxide stripping process, the excess ammonia and ammonium carbamate can be separated in a gaseous state at such a temperature as not to be extremely high even under relatively high pressure, for example, the urea synthesis pressure, so that the recovered ammonia and carbon dioxide can be recycled directly to the urea synthesis in the gaseous state. In addition to easiness of the recycle operation of the separated gas, heat recovery can be effected under the urea synthesis pressure which makes the temperature level advantageous. On the other hand, the separated gas can also be recycled to the urea synthesis either after condensing it or after absorbing it in a solvent such as water, a dilute aqueous ammonium carbonate solution, or an aqueous urea solution. Absorption under relatively high pressure such as the above requires a lesser amount of the solvent when the absorption is effected at an identical temperature, or requires about the same amount of the solvent even when the absorption is effected at a little higher temperature compared with the absorption in the separating operation under relatively lower pressure so as to be advantageous to both recycle operation and heat economy even though either absorption temperature may be chosen as above.
However, in the carbon dioxide stripping process, an intimate correlation between the stripping step and the composition of the urea synthesis effluent to be stripped, may make this process ineffective, depending on the composition of the urea synthesis effluent.
For example, when the ammonia concentration in the urea synthesis effluent to be stripped is too high, the contact of the urea synthesis effluent with carbon dioxide under heating first results in the absorption of carbon dioxide in the urea synthesis effluent instead of the decomposition of ammonium carbamate. Of course, the stripping is effected by bringing the urea synthesis effluent into countercurrent contact with carbon dioxide, and as the stripping operation proceeds, the amount of carbon dioxide absorbed in the urea synthesis effluent is gradually decreased to such an extent that the decomposition of ammonium carbamate finally proceeds. However, the decomposition step takes a considerably longer period of time to be completed, resulting in the necessity of enlarging the stripper and consequently, in operational and economical disadvantages.
In order to overcome the above problem, it is necessary to employ a urea synthesis effluent having a low concentration of ammonia, that is, a urea synthesis effluent obtained by a urea synthesis under the condition of a low ammonia to carbon dioxide molar ratio.
However, in the urea synthesis reaction represented by the following equation: EQU 2NH.sub.3 +CO.sub.2 .revreaction.NH.sub.2 CONH.sub.2 +H.sub.2 O
as an excess amount of ammonia is increased, the conversion of carbon dioxide to urea is increased, and conversely as the excess amount of ammonia is decreased, the conversion of carbon dioxide to urea is decreased. Accordingly, the decrease in the ammonia to carbon dioxide molar ratio decreases the conversion of carbon dioxide to urea and increases the amount of by-product ammonium carbamate based on the amount of urea obtained, resulting in increasing the load of the stripper for the decomposition of ammonium carbamate and in increasing the amount of heat required therefor. Although the heat used for the decomposition of ammonium carbamate is mostly recoverable as low pressure steam, the fact is that more valuable high pressure steam used in the stripper is recovered only as less valuable low pressure steam, resulting in being economically disadvantageous.